152 THE CELL 



water. Into this vacuole are secreted digestive enzymes which 

 reduce the ingested material if possible from the colloidal to the 

 crystalloidal state. It then passes into the protoplasm and 

 the undigested residue is forcibly excreted by contraction of the 

 vacuole. These processes all have their physico-chemical counter- 

 parts. A drop of chloroform will reject a piece of capillary glass 

 tubing forced into it. If the glass be coated with shellac it will 

 be drawn into the chloroform, the shellac dissolved from it and 

 then the clean glass be expelled from the interior of the chloroform 

 to the surface. 



{d) Growth is not a property characteristic of living matter. 

 Leduc has taught us that by osmosis life-like forms may be 

 produced which grow. 



[e) Electric Phenomena. The electrical power generated by 

 living matter has always been a subject of interest and of amaze- 

 ment. Quite apart from such animals as possess electrical organs, 

 e.g. the electric eel which can generate an E.M.F. of several 

 hundred volts, every living animal, in fact every living cell, pro- 

 duces electromotive forces. The ordinary potential differences 

 observed in living matter may seldom reach 0-1 volt, but everyone 

 knows that if 71 small units are connected in series, the resultant 

 voltage is n times the voltage of the single unit. Dissection of an 

 electric organ shows that it is built up serially of large numbers 

 of units. The cause of the potential differences in cells must be 

 sought for in the " selective " permeability of the cell membranes 

 (p. 145), or in alterations of the content of the protoplasm in 

 electrolytes. 



Electrical effects are produced in living cells by suitable 

 stinuilation. If a cell is injured the injured part becomes electro- 

 positive to the rest. This phenomenon, apart from any other 

 conditions, would be quite sufficient to justify the postulation of a 

 cell membrane. Consider the cell as a mass of protoplasm in an 

 envelope of matter which is permeable to the negative ion but 

 not to the positive ion of a dissociated electrolyte. This will 

 cause a difference of potential on the two sides of the membrane. 

 Inside will be a preponderance of negative ions while outside will 

 be an equal preponderance of positive ions (Fig. 38). Current of 

 Injury. — If now we could connect the inside of the membrane with 

 the outside, there would be a flow of current till the difference of 

 potential had been equalised. Current would flow to the pierced, 

 injured (or inside) part from the outside. That is, the injured 

 part would be similar to the zinc pole of a zinc-copper galvanic 

 couple. There the flow of current is from the zinc to the copper 

 inside the battery. The zinc is therefore said to be electro- 



